Abstract: A scheme using metamaterials is proposed to modulate beamwidth and obtain a line beam from a beam with finite width based on transformation optics, and the constitutive tensors of metamaterials are educed. The good performance of the proposed medium is verified through full-wave numerical simulations based on the finite element method. Such an electromagnetic wave-manipulating medium provides alternative ways either to expand or to compress the electromagnetic beamwidth. And the line beam with very high energy density at the beam center would find applications in enhancing nonlinear optical effects or other areas.